This invention relates to a process useful for oxidizing combustible components such as carbon monoxide generated during manufacture of sintered minerals.
In general, unburned carbon monoxide or CO generated during manufacture of sintered minerals, for example, is one of sources from which oxidative heat is to be recovered. The unburned CO cannot be oxidized at low temperatures because of its low concentration, and it is thus usually oxidized with the use of suitable catalysts.
However, exhaust gases from a sintering furnace generally contain a very minor proportion of poisoning substances which undesirably causes catalysts to be deteriorated. The catalyst deterioration used herein is represented by the rate of reduction in percent CO oxidation during passage of actual gases. The catalyst deterioration is reversible. To regenerate the catalyst, we devised regenerating systems as shown in FIG. 1 or FIG. 2 and filed patent applications, Japanese Patent Application Nos. 59-135188 and 5993773. The systems shown in FIGS. 1 and 2 are for treating exhaust gas 1 from a sintering furnace (not shown) after desulfuring and include a CO oxidation catalyst layer 2 or layers 3a, 3b, 3a'and 3b', dampers 4, 4a, 5, and 5a, rotary shafts with drive 6 and 6a, a denitrating reactor 7, or denitrating reactors 7a and 7b a heating furnace 8, a blower 9, a heat exchanger 10, and a stack 11.
The system of FIG. 1 is designed such that the gas flow is reversed when the catalyst is deteriorated to a given extent, and the system of FIG. 2 is designed such that the catalyst layers are reversed via a damper-like mechanism, both for the purpose of regenerating the catalyst.
These systems are required to reverse the catalyst layers or gas flow direction every one hour when they are operated at a catalyst layer inlet gas temperature of 390.degree. C. and a gas space velocity of 180,000 hr.sup.-1 because of the presence of trace amounts of poisoning substances in the exhaust gas to be treated from a sintering furnace.
There remains the need for further improving the above-proposed systems, particularly reducing the frequency of catalyst regeneration and the power consumption of an associated blower. These are the objects of the present invention.
More illustratively, an actual industrial installation requires control of the opening of dampers or the number of revolutions of the blower in the system in view of flow rate variation resulting from pressure fluctuation in the system during regeneration of the CO oxidizing catalyst as well as control of molar ratio of NH.sub.3 /NO.sub.x in view of the official prescribed limit of NO.sub.x. For positive operation and maintenance of an actual denitrating equipment, it is desired to reduce the frequency of catalyst regeneration.
The above-proposed systems can reduce the power consumption when applied to an actual industrial installation and the present invention provides a further reduction in power consumption. The present invention is intended to achieve reductions in frequency of catalyst regeneration and power consumption by optimizing the loading weight of platinum on a catalyst to thereby optimize the initial activity of the catalyst under sintering furnace exhaust gas conditions and to decelerate the rate of deterioration of the catalyst.
Generally, the loading weight of active ingredients in catalyst is determined in relation to the initial activity of the catalyst in most cases, and correlated to the rate of deterioration of catalyst in few cases.
As to the catalysts for oxidizing CO in exhaust gases from a sintering furnace, the loading weight of active ingredients is specified and described in Japanese Patent Publication No. 55-41812 and Japanese Patent Application Kokai Nos. 56-121643 and 54-1289. However, these noble metal loading weights are not determined in consideration of the rate of deterioration of the catalysts, and the specified or described loading weights are significantly lower than in the present invention.
More particularly, Japanese Patent Publication No. 55-41812 specifies the platinum loading weight to the wide range of from 0.0001 to 0.1% by weight on a carrier of metal material . The example of this Publication reports a platinum loading of 0.01% by weight, which is presumed to be 0.04 mg per square centimeter of the apparent outer surface area of the catalyst.
The term platinum loading weight per apparent outer surface area of catalyst is a platinum loading weight per geometric surface area of catalyst.
Japanese Patent Application Kokai No. 56-121643 discloses the optimum platinum loading weight of about 0.5% by weight for a catalyst comprising a carrier of TiO.sub.2 -SiO.sub.2 having platinum uniformly dispersed throughout the carrier. The platinum loading weight in the final catalyst is presumed to be 0.42% by weight based on the entire catalyst bulk and 0.13 mg per square centimeter of the apparent outer surface area of the catalyst. Since platinum is substantially uniformly dispersed throughout the catalyst, the loading weight of the platinum value on the catalyst outer surface contributing to the primary reaction is believed to be significantly lower than in the present invention. Of course, the platinum loading weight of the catalyst is not determined in relation to the rate of deterioration of catalyst.
Japanese Patent Application Kokai No. 54-1289 describes in Examples 1, 2, and 3 the loading weights of platinum of 1, 0.8, and 0.7 grams/liter among other noble metals on an aluminum carrier. The noble metal loading weight is presumed to be about 0.068, 0.77, and 0.067 mg per square centimeter of apparent outer surface area of catalyst in Examples 1, 2, and 3, respectively. Of course, the noble metal loading weight of the catalyst is not determined in relation to the rate of deterioration of catalyst.
The noble metal loading weight of the conventional CO oxidizing catalysts for sintering furnace exhaust gases is not at all determined in consideration of the rate of deterioration of catalyst which plays a great role in the process. Most of the noble metal loading weights described in these references, particularly the loading weights of the noble metal value present on the catalyst outer surface contributing to the primary reaction are significantly lower than in the present invention.
Loading of noble metal in such low weight is closely related to the initial activity of catalyst. When only the initial activity of catalyst is considered, noble metals such as platinum exhibit such a high oxidative activity that a sufficient initial activity is obtained with a small amount of noble metal added at the reaction temperature range of at least 300.degree. C. in the primary process as demonstrated in the following examples and other references to be listed hereinafter.
Shown below are noble metal loading weights in automobile emission catalysts in the prior art.
Automobile Technology, Vol. 35, No. 10 (1981), pages 1172-1176 describes low noble metal loading weights of 1.0 to 1.5 grams/liter for pellet catalysts (which presumably corresponds to 0.18 to 0.27% by weight and 0.10 to 0.15 mg per square centimeter of apparent outer surface area of catalyst) and 1.5 to 2.0 grams/liter for honeycomb catalysts (which presumably corresponds to 0.25 to 0.40% by weight and 0.068 to 0.091 mg per square centimeter of apparent outer surface area of catalyst). The present invention is significantly different from these automobile emission catalysts in the prior art in that the platinum loading weight of the honeycomb catalyst of the present invention is at least 2.4 grams/liter, and hence, at least 0.48% by weight or at least 0.20 mg per square centimeter of apparent outer surface area of catalyst.
Another example of automobile emission catalyst is disclosed in Japanese Patent Application Kokai No. 50-95188 as comprising a honeycomb structure carrier of cordierite which is coated with gamma-alumina and then impregnated with platinum and rhodium. It is described that Examples 1, 2, and 3 use noble metal loading weights of 0.54, 0.20, and 0.58% by weight, respectively, from which platinum loading weights of 0.42, 0.19, and 0.45% by weight of platinum are estimated.
In these conventional catalysts, no atttention has been paid to the relationship of noble metal loading weight to deterioration rate of a catalyst during sintering furnace exhaust gas treatment. The noble metal loading weight is set in consideration of the initial activity of catalyst. For this reason, the conventional catalysts have a lower noble metal loading than the catalyst of the present invention. When exhaust gases from a sintering furnace are treated with the conventional catalyst having such a low noble metal loading, neither reduction of the frequency of catalyst regeneration nor reduction in the thickness of a catalyst layer attributable to a retarded rate of deterioration is possible, and hence no reduction in power consumption is expectable.